Question 1

TTL noise immunity What is the standard noise margin per level for classic TTL logic (based on the usual VIH/VIL and VOH/VOL specifications)?

Accepted Answer

Introduction / Context: Noise margin quantifies how much unwanted voltage variation a logic level can tolerate without causing an error. TTL families publish typical thresholds and guaranteed output levels from which noise margins are derived. Given Data / Assumptions: VOH(min) ≈ 2.4 V, VOL(max) ≈ 0.4 V. VIH(min) ≈ 2.0 V, VIL(max) ≈ 0.8 V. Noise margin (high) = VOH(min) − VIH(min); noise margin (low) = VIL(max) − VOL(max). Concept / Approach: Compute each margin using given specs. High-level margin: 2.4 − 2.0 = 0.4 V. Low-level margin: 0.8 − 0.4 = 0.4 V. Therefore, the standard TTL noise margin per level is approximately 0.4 V. Step-by-Step Solution: Write the formulas for margins.Substitute the standard TTL values.Calculate both results = 0.4 V.Select 0.4 V as the answer. Verification / Alternative check: Family comparison charts consistently list TTL noise margins near 0.4 V, while CMOS at 5 V often has larger margins because VIH and VIL are fractions of VCC. Why Other Options Are Wrong: 5.0 V: That is the supply voltage, not noise margin. 0.0 V: Would imply no noise tolerance. 0.8 V: Does not match calculations from standard specs. Common Pitfalls: Mixing input and output thresholds or using typical instead of guaranteed values. Always base margins on guaranteed min/max, not typicals. Final Answer: 0.4 V

Question 2

Driving capability (fan-out) Approximately how many 74LS-TTL logic gate inputs can be reliably driven from the output of a single standard 74TTL gate?

Accepted Answer

Introduction / Context: Fan-out is the number of standard inputs a gate output can drive while meeting guaranteed logic levels. Because different TTL subfamilies have different input currents, the fan-out depends on both the driver family and the load family. Given Data / Assumptions: Driver: standard 74TTL output. Loads: 74LS-TTL inputs, which draw less current than standard TTL inputs. Use guaranteed VOL/VOH and IOL/IOH vs IIL/IIH numbers. Concept / Approach: Fan-out (LOW) ≈ IOL(max of driver) / |IIL(max per load)|; Fan-out (HIGH) ≈ IOH(max of driver) / IIH(max per load). 74LS inputs typically require much less current than standard TTL, so a standard TTL output can drive more LS inputs than standard TTL inputs. Typical textbook/handbook values round to about 20 LS loads per standard TTL output. Step-by-Step Solution: Identify current capabilities of the driver (IOL/IOH).Identify input currents of 74LS loads (IIL/IIH are small).Compute fan-out for HIGH and LOW; pick the limiting value.Result aligns with ≈ 20 LS inputs from a single 74TTL output. Verification / Alternative check: Reference logic charts in textbooks: standard TTL → fan-out 10 for standard TTL loads, and about 20 when driving LS inputs due to reduced current demand. Why Other Options Are Wrong: 10: Applies to standard-to-standard TTL loading, not LS loads. 200 / 400: Far exceed current limits; would violate VOL/VOH specs. Common Pitfalls: Ignoring both HIGH-state and LOW-state limits; true fan-out is the minimum of the two calculations. Also, temperature and supply variations can reduce margins. Final Answer: 20

Question 3

Handling unused TTL NAND inputs In practical circuit design, what should be done with unused inputs on a TTL NAND gate to ensure stable, low-noise operation?

Accepted Answer

Introduction / Context: Unused inputs can cause excess power, oscillation, or susceptibility to noise. TTL and CMOS families behave differently when inputs are left open; correct biasing depends on the logic technology. Given Data / Assumptions: Gate family: TTL (bipolar), device type: NAND. Goal: avoid floating inputs and undefined internal states. Use recommended bias practice from TTL datasheets. Concept / Approach: TTL inputs tend to float HIGH due to internal biasing but can pick up noise and switch unpredictably. Best practice is to tie unused TTL inputs to a defined logic level. For a NAND gate, tying unused inputs HIGH keeps them logically neutral, letting the used inputs control the gate output without added propagation or current issues. Step-by-Step Solution: Never leave TTL inputs floating; define a logic level.For NAND, a HIGH on an unused input does not change the function of the remaining inputs (X NAND 1 = NOT X).Tie unused inputs to VCC (directly or via a resistor as appropriate) to reduce noise.Verify output behavior remains predictable and stable. Verification / Alternative check: Application notes recommend tying unused TTL inputs to HIGH; for CMOS, unused inputs must also be tied to defined levels (often via pull-ups/pull-downs). Why Other Options Are Wrong: Let them float: risks oscillation and increased supply current. Tie LOW: For NAND, a LOW on any input forces output HIGH, potentially altering logic or increasing static current. Common Pitfalls: Confusing TTL and CMOS practices; while both should not float, the current behavior and preferred tie level can differ by gate function. Final Answer: tie them HIGH

Question 4

CMOS 74HC00 vs. 74HCT00 What is the principal difference between the 74HC00 and 74HCT00 series of CMOS logic devices?

Accepted Answer

Introduction / Context: Mixed-logic systems often combine CMOS and TTL devices. The 74HC (High-speed CMOS) and 74HCT (High-speed CMOS, TTL-compatible) series exist to ease interfacing by matching thresholds and levels across families. Given Data / Assumptions: Supply typically 5 V for compatibility discussions. TTL input thresholds: VIH(min) around 2.0 V, VIL(max) around 0.8 V. HC native CMOS thresholds center near VCC/2, which may not recognize TTL HIGH reliably at 5 V without margin. Concept / Approach: 74HCT devices modify input threshold circuitry to mimic TTL thresholds, ensuring a 2.0 V HIGH is recognized, while maintaining CMOS outputs compatible with TTL loads. This allows drop-in replacement of TTL in many designs without level translators. Step-by-Step Solution: Identify the need: TTL → CMOS interface without translators.Select HCT for TTL-compatible input thresholds.Confirm output levels and drive meet interfamily requirements.Choose the option that states TTL compatibility explicitly. Verification / Alternative check: Component datasheets label HCT inputs as TTL-compatible and show VIH/VIL matching TTL specs at 5 V. Why Other Options Are Wrong: “Faster/slower”: Speed differences vary by device; compatibility, not speed, is the defining feature. “Not TTL compatible”: Opposite of the HCT purpose. Common Pitfalls: Assuming HC inputs will always accept TTL highs; at lower VCC this may be fine, but at 5 V the thresholds can be marginal—use HCT. Final Answer: The HCT series is input and output voltage compatible with TTL.

Question 5

Meaning of “interfacing” in digital electronics In practical digital design, the term “interfacing” most often refers to:

Accepted Answer

Introduction / Context: Systems often mix logic families (TTL, CMOS, ECL), voltages (1.8–5 V), and signaling standards (LVCMOS, LVTTL, LVDS). “Interfacing” encompasses the techniques and circuits that translate and condition signals so that a driver's output meets the load's electrical requirements. Given Data / Assumptions: Driver and load may differ in voltage levels, current drive, polarity, or impedance. Requirements include DC levels, timing, impedance matching, and protection. Goal: reliable logic recognition with adequate margins and signal integrity. Concept / Approach: An interface circuit adapts one domain to another. Examples include level shifters (for example, 5 V TTL to 3.3 V CMOS), buffers/line drivers for fan-out and cable drive, and translators that convert single-ended to differential. Proper interfacing prevents overvoltage stress and logic errors. Step-by-Step Solution: Identify driver specs (VOH/VOL, IOH/IOL, rise/fall times).Identify load specs (VIH/VIL, input capacitance, impedance).Insert interface function (translator, buffer, resistor network) to satisfy both sides.Verify with timing and signal-integrity checks that margins are adequate. Verification / Alternative check: Application notes from major vendors provide selection guides for translators and buffers; lab measurements confirm correct levels and waveforms after interfacing. Why Other Options Are Wrong: Restricting to TTL-to-TTL (option a) is too narrow. TTL “operational amplifier” (option c) is a misnomer. Input buffer only (option d) does not cover the full meaning of interfacing. Common Pitfalls: Ignoring directionality, assuming 5 V-tolerant inputs where none exist, or forgetting pull-ups/pull-downs for open-collector/open-drain lines. Final Answer: A conditioning circuit between driver and load to ensure compatibility.

Question 6

CMOS driving TTL — matching current and logic-level requirements When CMOS logic circuits must drive TTL logic circuits, different current requirements often cause interface issues. Which practical addition is commonly used to overcome this mismatch…

Accepted Answer

Introduction / Context: Interfacing different logic families—specifically CMOS outputs driving TTL inputs—can fail if the driver cannot source/sink the input currents that the receiver expects. This question focuses on the practical hardware addition that restores noise margin and guarantees valid HIGH/LOW levels when CMOS must drive TTL loads. Given Data / Assumptions: CMOS outputs (especially older 4000-series) have limited source/sink current compared to TTL requirements. TTL inputs draw more input current (notably IIL when low) and have VIH(min) near 2.0 V to 2.4 V for standard TTL. Goal: achieve reliable logic-level compatibility and drive strength without redesigning the entire system. Concept / Approach: Adding a buffer stage tailored to the source family solves both current drive and edge-rate issues. A CMOS buffer or inverting buffer (e.g., 4049/4050-type or HC/HCT/ACT family buffers) provides higher fan-out and sharper transitions, ensuring that TTL VIH/VIL thresholds are met under load. If HCT or dedicated “TTL-compatible” CMOS buffers are used, their input thresholds are designed to accept TTL levels while their outputs can drive multiple TTL inputs. Step-by-Step Solution: Identify the mismatch: CMOS cannot comfortably drive multiple TTL inputs due to current and threshold differences.Insert a CMOS buffer/inverter with adequate drive (preferably TTL-compatible variants like HCT).Verify that VOH/VOL and rise/fall times meet TTL input specifications.Confirm improved fan-out and margin across temperature and supply variations. Verification / Alternative check: Consult buffer datasheets for IOH/IOL and VOH/VOL specs at desired fan-out. Lab measurements with an oscilloscope will show restored edge integrity and margins. Why Other Options Are Wrong: A and C: “Bilateral switches” are analog pass devices, not ideal for robust digital level translation or current drive. B: A TTL tristate buffer may work in some cases, but the question generalizes CMOS→TTL; a CMOS buffer solution is the standard recommendation. Common Pitfalls: Ignoring input thresholds (TTL vs. CMOS) and relying only on pull-up resistors; this can degrade edges and noise immunity at higher speeds. Final Answer: a CMOS buffer or inverting buffer

Question 7

Key advantage of ECL (Emitter-Coupled Logic) Among common digital logic families, which major advantage is most strongly associated with ECL technology?

Accepted Answer

Introduction / Context: Different logic families trade off speed, power, and complexity. Emitter-Coupled Logic (ECL) is well known in high-performance systems such as telecom and test equipment. This question asks for the signature advantage of ECL that motivates designers to use it despite its power cost. Given Data / Assumptions: ECL operates with constant current sources and differential pairs. Transistors are kept out of saturation to avoid storage delay. Application priority is speed rather than power economy. Concept / Approach: ECL’s architecture minimizes delay by steering current between differential transistor pairs with small voltage swings. Since devices do not saturate, they avoid the charge removal time that slows other bipolar families. The result is extremely fast propagation delays compared with TTL/CMOS of similar vintage, at the expense of higher static power dissipation and more careful power/ground design. Step-by-Step Solution: Identify ECL characteristics: non-saturating, differential, constant current.Relate these to performance: very short propagation delays.Select the advantage that directly follows: very high speed.Acknowledge trade-offs: higher power and tighter analog considerations. Verification / Alternative check: Classic ECL families (e.g., 10K/100K) demonstrate sub-ns to a few-ns delays, historically outperforming TTL and many CMOS families at the time. Why Other Options Are Wrong: B: ECL does not offer a wide operating voltage range; supplies are constrained (often negative rails). C: ECL is not low-cost compared with mainstream CMOS/TTL. D: High power is a drawback, not an advantage. Common Pitfalls: Assuming the fastest family is always best; system-level power and thermal budgets may favor CMOS. Final Answer: very high speed

Question 8

Logic-family interfacing safety: Evaluate the claim: “Interfacing between 74HCMOS and 74ALS/74LSTTL, or between 74TTL and 74LSTTL, can always be done with no danger.” Decide whether this statement is accurate for logic-level compatibility and drive.

Accepted Answer

Introduction / Context: Interfacing different logic families requires checking both logic-level compatibility (VIH/VIL versus VOH/VOL) and drive capabilities (fanout, input currents). Blanket statements about “no danger” are risky. This question probes recognition of level compatibility nuances among 74HC, 74HCT, TTL, ALS, and LSTTL families. Given Data / Assumptions: 74HC CMOS has CMOS-level VIH requirements (~0.7*VCC), not TTL-compatible. 74HCT is TTL-compatible (HCT “T” = TTL input thresholds). 74ALS/74LSTTL are TTL families with defined VOH/VOL; VOH(min) may be ~2.4 V. Device currents and fanout must be respected; some TTL outputs source weak HIGH current. Concept / Approach: Safe interfacing demands that VOH/VOL of the driving family meet VIH/VIL of the receiving family with comfortable noise margins. 74HC inputs often need VIH near 3.5 V at VCC=5 V; a TTL output VOH(min) at ~2.4 V will not reliably satisfy that. 74HCT fixes this by using TTL thresholds. Therefore, the absolute claim of “no danger” is wrong. Step-by-Step Solution: Compare TTL VOH(min) (~2.4 V) with 74HC VIH(min) (~0.7*5 V = 3.5 V) → incompatibility risk.Recognize 74HCT inputs are TTL-compatible → OK with TTL/ALS/LSTTL drivers.Infer: Interfacing is conditionally safe, not “always with no danger.” Verification / Alternative check: Check datasheets for VIH/VIL/VOH/VOL and ensure positive noise margins in both directions. Why Other Options Are Wrong: Correct: Would ignore documented threshold mismatches.Only correct if using 74HCT instead of 74HC: This is a helpful guideline but not universally sufficient; drive currents and fanout still matter.Only correct at very low frequencies: Frequency does not fix DC level mismatches. Common Pitfalls: Assuming “HC = HCT” for thresholds.Ignoring fanout and input leakage/drive limits. Final Answer: Incorrect

Question 9

TTL implementation insight: Assess the statement: “The AND gate is the simplest to implement in TTL and therefore requires the least circuitry.” Consider typical TTL gate realizations.

Accepted Answer

Introduction / Context: In TTL logic, the primitive most directly realized at the transistor level is typically NAND (or NOR in some families), not AND. Recognizing which primitive is “simplest” affects how complex functions are built and how many packages a design needs. Given Data / Assumptions: TTL families commonly provide NAND gates as fundamental building blocks. An AND function generally requires a NAND followed by an inverter (or an equivalent transformation). Complexity is judged by transistor count and stage count, not just the name of the logic operation. Concept / Approach: Because TTL input stages and totem-pole outputs lend themselves naturally to NAND, AND is not the simplest. Implementing AND often involves extra inversion, which increases transistor count and propagation delay. Step-by-Step Solution: Identify primitive → NAND is most natural in TTL.Implement AND → NAND followed by inversion (or De Morganʼs transformations).Therefore AND is not the simplest, contradicting the statement. Verification / Alternative check: Standard TTL families (74xx) offer plentiful NAND packages; AND versions exist but typically incur extra stages internally. Why Other Options Are Wrong: Correct: Would misrepresent TTL gate economics.Only correct for Schottky TTL: Schottky speeds up devices but does not change the primitive.Only correct at low fan-in: Primitive preference remains NAND. Common Pitfalls: Equating boolean simplicity with hardware simplicity.Ignoring internal inversion required for AND in TTL. Final Answer: Incorrect

Question 10

Propagation delay in TTL: Judge the statement: “Propagation delay in TTL is due to slow switching speeds.” Consider device physics like transistor storage and capacitances.

Accepted Answer

Introduction / Context: Propagation delay is the time from an input transition to the corresponding valid output transition. In TTL devices, this delay arises from the finite switching dynamics of transistors and the charging/discharging of internal and load capacitances. Given Data / Assumptions: TTL uses multi-transistor stages (input transistor network, phase splitter, totem-pole output). Each transition involves storage and removal of charge in junctions. External loading also adds capacitance that must be driven. Concept / Approach: “Slow switching speed” is a lay description of the physical causes: transistor storage time, finite carrier recombination, and RC time constants. All contribute to non-zero rise/fall times and internal stage delays, producing an overall propagation delay specification (e.g., 10 ns to tens of ns depending on the family). Step-by-Step Solution: Identify propagation delay sources → device internal switching + load effects.Recognize that faster processes (e.g., Schottky clamps) cut storage time, reducing delay.Therefore, propagation delay is indeed due to finite (not instantaneous) switching → statement is correct. Verification / Alternative check: Datasheet timing confirms lower delays for faster variants (e.g., 74S, 74AS) due to improved switching elements. Why Other Options Are Wrong: Incorrect: Ignores well-understood transistor switching physics.Only correct for LS families / high temperature: Delay varies with family and temperature but the underlying cause remains finite switching. Common Pitfalls: Treating propagation delay as unrelated to switching; they are directly linked.Assuming logic-level compatibility affects intrinsic delay; it primarily affects interfacing. Final Answer: Correct

Question 11

Signal edge realism: Evaluate the statement: “A digital pulse is not perfectly square; it requires finite time to rise from 0 to 1 and to fall from 1 to 0.”

Accepted Answer

Introduction / Context: Ideal square waves have instantaneous transitions. Real digital circuits, however, exhibit finite rise and fall times due to device physics and parasitic capacitances. Recognizing this helps with timing analysis, signal integrity, and bandwidth considerations. Given Data / Assumptions: Any physical driver has limited slew rate. Interconnects add capacitance and inductance. Loads present capacitance that must be charged/discharged. Concept / Approach: The rise time (tr) is commonly defined between 10% and 90% of the final value; fall time (tf) is similarly defined. Nonzero tr/tf means the waveform is not perfectly square and has finite spectral content, which affects ringing, overshoot, and EMI. Step-by-Step Solution: Recognize physical limitations → drivers cannot change voltage instantaneously.Account for RC + L effects → interconnect parasitics slow edges.Conclusion → pulses are not perfectly square; finite rise/fall times are universal. Verification / Alternative check: Oscilloscope measurements always show nonzero tr/tf; datasheets specify typical/max rise/fall times. Why Other Options Are Wrong: Incorrect: Contradicts both measurement and theory.Only correct above 10 MHz / only for CMOS: Edge finiteness applies to all frequencies and families, including TTL and ECL. Common Pitfalls: Assuming logic “1/0” ideals hold exactly in time domain.Ignoring the impact of edge rate on EMI and transmission-line effects. Final Answer: Correct

Question 12

TTL noise margins clarification: Assess the claim: “In TTL the noise margin is between 0.8 V and 0.4 V.” Use standard TTL logic-level thresholds to determine correctness.

Accepted Answer

Introduction / Context: Noise margin quantifies the amount of noise a logic signal can tolerate without causing a logic error. For TTL, you determine noise margins by comparing guaranteed output levels (VOH/VOL) to required input thresholds (VIH/VIL). Given Data / Assumptions: Typical TTL guaranteed levels: VIH(min) ≈ 2.0 V, VIL(max) ≈ 0.8 V. Typical TTL outputs: VOH(min) ≈ 2.4 V, VOL(max) ≈ 0.4 V. Noise margin high (NMH) = VOH(min) - VIH(min) ≈ 2.4 - 2.0 = 0.4 V. Noise margin low (NML) = VIL(max) - VOL(max) ≈ 0.8 - 0.4 = 0.4 V. Concept / Approach: The statement says “between 0.8 V and 0.4 V,” which is ambiguous and misleading. TTL noise margins are not a range bounded by 0.8 V and 0.4 V; instead, both the high and low noise margins are about 0.4 V each under standard specs. Step-by-Step Solution: Compute NMH → 2.4 - 2.0 = 0.4 V.Compute NML → 0.8 - 0.4 = 0.4 V.Conclusion → Both margins are ~0.4 V, not “between 0.8 and 0.4.” Verification / Alternative check: Check any standard 74xx TTL datasheet; values are consistent across subfamilies, with variations in advanced families. Why Other Options Are Wrong: Correct: Would endorse an imprecise/incorrect characterization.Only correct for open-collector TTL / VCC = 3.3 V: Noise margins are defined by thresholds, not collector configuration alone, and classic TTL is 5 V. Common Pitfalls: Confusing threshold levels (0.8 V and 0.4 V) with noise margins themselves.Failing to compute NMH/NML explicitly. Final Answer: Incorrect

Question 13

Technology usage statement: Evaluate the claim: “PMOS and NMOS are commonly used for small memories and microprocessors.” Consider historical and modern mainstream processes.

Accepted Answer

Introduction / Context: MOS technologies include PMOS, NMOS (single-channel processes), and CMOS (complementary MOS). Historically, early microprocessors and small memories used NMOS; PMOS was used even earlier. Modern mainstream digital ICs overwhelmingly use CMOS due to power and density advantages. Given Data / Assumptions: Legacy designs (1970s–early 1980s) often used NMOS for microprocessors and memories. CMOS supplanted NMOS/PMOS because it offers low static power and high integration. The statement is assessed in a general, current-technology context. Concept / Approach: “Commonly used” should reflect current industry practice. Today, small memories (SRAM, ROM, Flash) and microcontrollers/processors are fabricated in CMOS logic processes (or mixed-signal CMOS variants). PMOS-only or NMOS-only processes are not the common choice for mainstream products. Step-by-Step Solution: Identify historical vs. modern → NMOS/PMOS were common historically; modern practice is CMOS.Assess the phrase “commonly used” → in modern practice, false for PMOS/NMOS-only processes.Therefore, the statement is incorrect in contemporary context. Verification / Alternative check: Review any current foundry PDK catalog; mainstream digital offerings are CMOS-based nodes. Why Other Options Are Wrong: Correct: Would misstate current industry norms.Correct only for legacy 1970s designs: While historically true, the prompt asks about general/common usage today, hence overall incorrect.Correct only below 1 kb density: Technology preference is not dictated solely by tiny memory size; CMOS still dominates. Common Pitfalls: Assuming a historically accurate statement remains true today.Confusing device-level PMOS/NMOS transistors with single-channel process technologies. Final Answer: Incorrect

Question 14

Emitter-Coupled Logic (ECL): Evaluate the claim — “In ECL, the logic HIGH and logic LOW levels are determined by which transistor in the differential amplifier pair conducts more current.”

Accepted Answer

Introduction / Context: Emitter-Coupled Logic (ECL) is a high-speed bipolar logic family that avoids transistor saturation by operating a differential amplifier stage. The heart of ECL is a long-tailed pair whose two transistors steer current between two collector load paths. This question tests whether you understand that ECL logic states (HIGH/LOW) arise from which side of the differential pair is conducting more, creating distinct voltage levels at the collectors and at the following emitter-follower outputs. Given Data / Assumptions: An ECL gate core is a differential pair referenced to a temperature-compensated emitter-referenced bias (Vref). Only one side (more precisely, predominantly one side) of the pair conducts at a time. Collector loads translate current steering into voltage differences, then emitter followers shift and buffer the levels. Concept / Approach: ECL works by comparing an input to a reference using a differential pair. If the input exceeds the reference, one transistor draws more current; if it is less, the other side conducts. Because neither side saturates, charge storage is minimal and switching is very fast. The output logic levels are, therefore, a direct consequence of which branch of the differential pair is conducting more current through its load resistor(s). Step-by-Step Solution: Identify the differential pair as the decision element in ECL.Recognize that input vs. reference biases one transistor more than the other.More current in a branch produces a lower collector voltage on that branch (due to drop across the load), which then appears as a specific logic state after the emitter follower.Thus, logic HIGH/LOW correspond to which transistor draws more of the tail current. Verification / Alternative check: Sketch a basic ECL OR/NOR gate: the differential pair drives two collector nodes with load resistors; the higher-conducting side pulls its collector lower. Emitter followers add level shifting to standard ECL logic levels (negative rail referenced), confirming the mechanism. Why Other Options Are Wrong: Incorrect: Contradicts the fundamental differential-pair operation. True only if loads are matched: Matching improves symmetry, but the mechanism still holds with typical tolerances. Cryogenic temperature requirement: ECL works over normal ranges; no cryogenic operation is required. Only for emitter followers: The emitter follower buffers the result; the decision comes from the differential pair. Common Pitfalls: Confusing the emitter-follower buffer with the decision element; assuming saturation-based logic like TTL; overlooking the role of the bias reference Vref in setting switching thresholds. Final Answer: Correct

Question 15

IC numbering and standardization: Evaluate the claim — “Basic part numbers of digital logic ICs are consistent across manufacturers because families (e.g., 74xx) are standardized.”

Accepted Answer

Introduction / Context: Digital logic families (TTL, CMOS derivatives) are cataloged using well-known numbering schemes such as the 74xx series. This question checks your knowledge that a given base part number (for example, 74LS00 as a quad 2-input NAND) denotes the same logic function across different manufacturers, even if process details and speed/power grades vary. Given Data / Assumptions: Manufacturers follow JEDEC/industry conventions for logic functions. The prefix/suffix may denote technology (LS, HC, HCT, ALS) or package/temperature grade. Electrical specs can vary slightly by vendor, but function and pinout match. Concept / Approach: Standardization arose to ensure interoperability and second sourcing. While timing, power, and absolute maximum ratings can differ, a 74HC00 is functionally a quad NAND with standardized pinout regardless of vendor. This enables drop-in replacements in many cases. Step-by-Step Solution: Identify that “basic part number” maps to a logic function (e.g., 00 = NAND, 04 = inverter).Recognize that multiple vendors supply the same base numbers with compatible pinouts.Conclude the statement is correct, with caveats regarding grades and specs. Verification / Alternative check: Compare datasheets from different vendors for the same 74xx code; the truth tables and pinouts align, confirming standardization. Why Other Options Are Wrong: Incorrect / vendor-unique: Contradicted by decades of cross-vendor pin/function compatibility. CMOS-only or military-only: Standardization spans TTL and CMOS families and different grades. Common Pitfalls: Ignoring subtle differences (drive strength, speed grade); overlooking that 54xx vs. 74xx denotes temperature/mil spec differences. Final Answer: Correct

Question 16

Schottky TTL concept check: Evaluate — “Schottky logic mitigates saturation and stored charge by placing a Schottky diode across the base–collector junction.”

Accepted Answer

Introduction / Context: Standard TTL can suffer from transistor saturation and storage delay. Schottky TTL (S-TTL and its low-power variants) adds Schottky clamp diodes between base and collector to prevent deep saturation, effectively speeding up turn-off and improving propagation delay. Given Data / Assumptions: Schottky diode forward voltage is lower than a silicon PN junction. Clamping the base–collector voltage prevents the transistor from saturating. Reduced charge storage yields faster switching. Concept / Approach: The Schottky diode forms a clamp so that when the transistor attempts to drive into saturation, the Schottky diode conducts first, keeping Vbc low and preventing stored charge accumulation. This allows rapid removal of carriers and quicker transition to cutoff. Step-by-Step Solution: Note the problem: saturation causes storage delay.Add a Schottky clamp across base–collector to limit Vbc.Clamping prevents saturation; device switches off faster; thus the statement is correct. Verification / Alternative check: Datasheets for 74Sxx/74LSxx families highlight improved speed versus standard 74xx due to Schottky clamping. Why Other Options Are Wrong: Incorrect / emitter–base: The classic clamp is base–collector, not emitter–base. ECL-only: ECL avoids saturation differently; Schottky TTL is specific to TTL improvements. Decoupling: Helpful for noise, not a substitute for anti-saturation clamps. Common Pitfalls: Confusing clamp location; assuming any diode helps; overlooking that Schottky devices are metal–semiconductor junctions with low forward drop. Final Answer: Correct

Question 17

TTL fan-out reality check: Evaluate — “A typical fan-out for most TTL devices is 9.”

Accepted Answer

Introduction / Context: Fan-out in TTL is the number of standard inputs a gate output can reliably drive while meeting logic-level specifications. Historically, standard TTL families specify a fan-out of about 10 in the LOW state (sinking current), a widely quoted rule of thumb in textbooks and datasheets. Given Data / Assumptions: Standard TTL (e.g., 74xx, 74LSxx) defines input current requirements and output drive capability. Fan-out is calculated as I_OL(max sink) / I_IL(max per input) for LOW, and I_OH(max source) / I_IH(max per input) for HIGH. Many TTL families arrive near 10 for LOW-state drive. Concept / Approach: Typical TTL fan-out is taught as 10, not 9. Some variations exist by subfamily and temperature, but “9” is not the standard nominal. Therefore the claim is inaccurate in general contexts. Step-by-Step Solution: Check TTL data: a driver can sink roughly 16 mA at VOL spec, each input may require about 1.6 mA.Compute LOW fan-out ≈ 16 mA / 1.6 mA = 10.Thus the commonly accepted value is 10, not 9 → statement is incorrect. Verification / Alternative check: Consult standard 74xx and 74LSxx datasheets; many show IOL = 16 mA and IIL = 1.6 mA for the reference load, yielding fan-out ≈ 10. Why Other Options Are Wrong: Correct / subfamily/voltage/rise-time qualifiers: These specifics do not shift the canonical figure to 9 as a typical value. Common Pitfalls: Confusing worst-case margins or particular device variants with the widely taught nominal; ignoring that HIGH-state fan-out can differ from LOW-state fan-out. Final Answer: Incorrect

Logic Families and Their Characteristics Questions